Overview
The present invention relates to communications services using the power distribution network. The following provides a general background of a power generation system used to supply power to the power distribution network, general background of a power transmission system, a power transmission system, a power distribution network, as well as background information concerning power line communications; that is, the use of the power distribution network to deliver information.
Power Generation System
Power is provided to users worldwide through the use of three distinct systems. A power generation system, typically located at a power generation facility, converts some form of potential or kinetic energy into electricity through the use of electrical generators. These generators are typically powered by combustion, hydroelectric, wind, or nuclear power sources.
Power Transmission System
The power transmission system is typically electrically connected to a power generation system, and delivers the generated electricity over a large distance from the Generation system to the localities where that electricity is consumed. The power transmission system consists of transformers which, from the generated power, produce High Voltage (HV), known in the art to be voltages from 60,000 to 1,000,000 volts. The Transmission System may carry either Alternating Current (AC) or Direct Current (DC), and typically uses what are known as “high tension” electrical wires (cable) for transmission. The power transmission system typically ends at one or more power substations, which are geographically close to the consumers of the electricity. Power transmission systems use large transformers, overload and lightning protection devices, switches, and various network sensing and control devices. Power Transmission wires are typically overhead, and the electrical wires are usually fabricated from un-insulated aluminum.
Power Distribution Network
A power distribution network is a series of electrical wires (cables) and components used to deliver power from the substation to the individual consumers of electricity. Distribution networks typically use Medium Voltage (MV), known in the art to be from 4,000 to 50,000 volts, and almost always use AC. Located in locations convenient to the power supplier, transformers connect to the MV Power Distribution System and produce Low Voltage (LV) electricity at from 90 to 600 volts AC, which is delivered to the consumers of the electricity. A single LV transformer may power one customer, several customers, or hundreds of customers. Power Distribution systems use transformers, switches, reclosers, lightning and fault protection devices, capacitors, meters, and other sense and control devices. Power Distribution wires may be overhead, where they may be insulated or un-insulated. They may also be underground wires (cables), which typically contain a center power conductor surrounded by one or more ground leads in a coaxial arrangement.
Power Distribution Networks are often deployed in a tree-like topology, with the root of the tree located at the substation, and the major trunks known as “feeders” extending from this location. Each feeder in turn has multiple branches, also known in the art as “laterals”, which extend from the feeders outward. A lateral may in turn feed several other laterals. The feeders and laterals often extend 25 km (15.54 miles) or more from the substation.
Very often the “leaves” of the tree, or outermost laterals, are arranged in geographic loops, such that there are multiple paths to any consumer from the substation. Because these loops produce a safety and fault detection and correction problem, they are typically opened in one place by an automatic switch or a manual Normally Open Point. In the event of an outage, the normally open point can be closed, providing an alternate electricity flow path and reducing the number of consumers effected by the fault condition.
Power Line Communications (PLC) is the art of re-using the Power Transmission System and Power Distribution Network for the delivery of information. As is known in the art, PLC system are divided into two categories: Customer Premises (CP) networks, which operate completely on LV power within the electric consumer's premises; and Access networks, which operate on the Transmission and/or Distribution networks, at HV or MV. PLC systems and devices superimpose an information signal on 50 or 60 Hz power signal, such that the power distribution devices are unaffected by the additional signal.
PLC may use narrowband or broadband data transmission. Narrowband PLC has been in use since the 1970's for transmission of control and sense signals across the Transmission and Distribution networks, by and for the electric utility. Very often these systems generate high-frequency pulses during the 50 or 60 Hz zero-crossing period, and use these pulses or their absence to carry information (e.g. electrical utility meter readings, etc.).
Broadband PLC, which transports e.g. 1 Mb/s or more of information, typically uses spread-spectrum or frequency-hopping techniques. Such techniques are used because power lines do not readily transport signals above the 50 or 60 Hz for which they were designed. Higher frequencies are attenuated quickly, and overhead wires particularly are very noisy, carrying radio and television signals, as well as other narrowband and broadband noise. Because governmental electromagnetic interference regulations prohibit simply increasing the PLC power levels above those noise levels, PLC modulation schemes typically are designed such that they statically or dynamically avoid the noise. Additionally, the signals may be adjusted manually or automatically to counteract the signal attenuation introduced by various components of the Transmission or Distribution system.
PLC is sometimes used to carry packetized data, using protocols such as the Internet Protocol (IP), the Transmission Control Protocol (TCP), and the User Datagram Protocol (UDP). Other protocols, such as Appletalk, may also be used. In these cases, the PLC network may operate in the data realm as a collection of data forwarding elements including repeaters (which operate in what is known in the art as the International Standards Organization (ISO) Open System Interconnection (OSI) reference model, layer 1, bridges or data switches, which operate at OSI layer 2, routers, which operate at OSI layer 3, or gateways, which operate at OSI layers 4-7.
There is no constraint that the data network is topologically aligned to the power Transmission or Distribution network. For example, a data signal may well traverse the power lines from a lateral or feeder toward a substation, which power would never do. Similarly, even if the superimposed PLC data network were arranged in a tree topology, the root node may or may not correspond to the root node of the Power Distribution System (the substation). The PLC network may in fact use loops or complex mesh topologies, using protocols known in the art, such as the International Electrical and Electronic Engineering (IEEE) 802.1D spanning tree, the Routing Information Protocol (RIP), or the Open Shortest Path First (OSPF) protocol to determine a packet's path across the PLC network.
PLC networks may also transport un-packetized voice or video streams, such as those used for telephony or cable television systems. Telephony streams may be formatted using standard telephony framing methods, commonly known in the art as T1, E1, or Synchronous Optical Network (SONET) framing. Video signals may use modulation, encoding, and framing techniques such as National Television System Committee (NTSC), Digital Video Broadcast (DVB), or Moving Picture Experts Group (MPEG).
PLC
As stated above, high speed RF (radio frequency) communications, such as broadband, can be implemented over the medium voltage electric power lines of a power distribution network, subject to many constraints. As can be seen from FIG. 1, the physical topology of the network resembles the branching of a tree. The root of the tree corresponds to the medium voltage feeder as it leaves the power transmission system substation, or the point on a branch where it connects to the feeder. The feeder is a three-phase, three-wire power line with a ground conductor. At various intervals down the line, branches leave the main feeder to further distribute the power. The branches may be three-phase, any two of the three phases, or any single phase. Branches are rarely terminated, other than by the last transformer or other power distribution component needed for the delivery of power. Various switching components (including manual switches, automated reclosers and sectionalizers, as well as fuses) are inserted at appropriate locations to manage line faults and control the distribution of power. The squares labeled “R” and “S” in FIG. 1 are “Recloser” and “Sectionalizer”. Reclosers and sectionalizers are typical automatic switch-gear. Other components (capacitors and regulator transformers) may be present to adjust the power factor or voltage levels along the line. Any of these components may present barriers to propagation of the RF signals, and may require additional devices to create RF bridges around them. The length of the power line from the root to the most distant branch tip may be 25 km or more. It appears very likely that some network stations will not be able to “hear” other stations in the network because of the attenuation of RF signals over a large distance between stations, or the RF barriers along the power line. This last point implies that a peer-to-peer network architecture is not appropriate for this system.
In a typical power distribution network, loops may be formed by switch closures made to restore lost power to an area, or for other reasons such as load balancing or line redundancy. All along the MV lines, distribution transformers convert the 4-30 KV voltage levels of the Medium Voltage power lines down to 110-600 V range of the Low Voltage (LV) power lines.
The power line environment, especially when using overhead lines, is electrically noisy, with many narrowband noise sources, and significant broadband noise. A communications system functioning in this environment must use every practical means of improving its noise resistance.
It is expected that the power line noise and RF transmission characteristics may change with the weather, as well as with the degree and kind of electrical loads connected to the line. The topology of any particular power line may produce unique reflection patterns or resonance conditions that may degrade the medium with respect to communications usage. It follows that the MV power line communication system must be adaptable to changing environmental characteristics, and must not be dependent on any single frequency (i.e., it cannot be a narrow-band system.)
The primary function of the power line is to deliver power. The communications network may not compromise that function. Thus, no communications device may be inserted into the line, breaking any of the power conductors. It follows that it may be technically challenging to produce coupling devices which introduce RF signals onto the power medium and extract RF signals from the medium. It may also be difficult to isolate RF signals on the power line from each other, as the medium may not be conducive to the filtering of high frequency signals, and as such low level protocols must be able to identify and reject traffic leaking into a network segment from other nearby segments.
Data communications over the power line are bi-directional, and must function over the three-phase line as well as any single-phase line. This constraint implies that communications reverse directions on the line in a time dependent manner (used as a truly half-duplex medium), or the medium is used in a full-duplex mode, with frequency domain multiplexing to provide the required bi-directionality.
The principal function of the MV power line RF network is that of an access network. Customers subscribe to communications access as a means to reach the Internet, or as a means to implement a virtual private network over a shared medium. The MV power line network is not a local area network.
It is advantageous and desirable to provide a broadband communications service over the medium voltage distribution network while meeting the above-described constraints.